Device density in integrated circuits (ICs) is constantly being increased. To enable the increase in density, device dimensions are being reduced. As the dimensions of device contacts get smaller, device contact resistance increases, and device performance is adversely affected. Methods for decreasing device contact resistance in ICs are needed to obtain enhanced device and IC performance.
Device contacts with reduced resistance may be created by forming certain metals on a silicon semiconductor base layer. These metals react with the underlying silicon, for example, to form silicides. Silicide device contacts are desirable because they reduce the native oxide on silicon. The native oxide is undesirable because it increases the contact resistance.
In one embodiment, titanium is used to form silicide device contacts for two reasons. First, titanium silicide has superior gettering qualities. Also, titanium silicide forms low resistance contacts on both polysilicon and single-crystal silicon.
Titanium silicide device contacts are normally formed with the following process. First, a thin layer of titanium is formed on top of the silicon base layer, such as a substrate. The titanium adjoins active regions exposed by contact holes in an isolating layer, such as an oxide, above the silicon base layer. Then, the silicon base layer is annealed. As a result, the titanium reacts with the active regions of silicon to form titanium silicide.
However, because titanium cannot be readily deposited in a pure form, additional processing steps are required to form titanium silicide device contacts. Titanium precursors, such as titanium tetrachloride, are commonly available and can be used to form titanium. Titanium tetrachloride, though, can only be reduced at temperatures exceeding 1000 degrees Celsius with certain reducing agents. At these temperatures, the silicon base layer will be damaged. Therefore, there is a need for a method of forming titanium from titanium precursors at lower temperatures.
Furthermore, the resistance of device contacts can be adversely increased by conductive layers coupled between the device contacts and other components. The conductive layers may be formed by the same metal layer used to form the device contacts. As device dimensions shrink, the contact holes become relatively deeper and narrower. Also, the walls of the contact holes become steeper, and closer to vertical. As a result, most metal deposition techniques form conductive layers having relatively small step coverage, and hence relatively high resistance. Step coverage is the ratio of the minimum thickness of a film as it crosses a step, to the nominal thickness of the film on flat regions, where thickness is generally measured perpendicular to the surfaces of the step and flat regions, and where the resultant value is usually expressed as a percentage. Thus, the effective contact resistance is increased at lower values of step coverage. Therefore, there is also a need for a method of forming conductive layers having increased step coverage to reduce effective device contact resistance.
Conformal layers of titanium having good step coverage have been previously formed at lower temperatures with chemical vapor deposition. Such techniques are disclosed in U.S. Pat. Nos. 5,173,327, 5,273,783 and 5,278,100, which are hereby incorporated by reference. However, alternative, effective and efficient techniques for forming titanium films are desired.